1. Field of the Invention
The present invention relates to color liquid crystal displays. In particular, the present invention relates to a black matrix for a color liquid crystal display widely used for lap-top computers and portable televisions.
2. Description of the Related Art
FIG. 11 is a cross-sectional view of a typical TN (Twisted Nematic) type color liquid crystal display having an active matrix drive. As shown in FIG. 11, a color liquid crystal display basically has a structure in which a liquid crystal material 3 is enclosed between two transparent substrates 1 and 2. The inner face of one transparent substrate 1 forms a transparent electrode 11 for a TFT (Thin Film Transistor) device, and the inner face of another transparent substrate 2 forms a color filter 21 comprising three color pixels, i.e., red, green and blue. On the surface of the color filter 21, another transparent electrode 22 is formed crossing the transparent electrode 21.
Polarizing films 110 and 210 are provided on the outer faces of the transparent substrates 1 and 2. The polarizing difference between the polarizing films 110 and 210 is a 90.degree. angle. A back light (not shown) is provided outside the transparent substrate 1 as a light source.
When turning off the TFT device, the light passing through the polarizing film 110 from the back light is twisted to a 90.degree. angle along the molecular orientation of the liquid crystal 3. The 90.degree. angle-twisted light passes the polarizing film 210 through the color filter 21 and the transparent substrate 2 in series. On the other hand, when turning on the TFT device, liquid crystal molecules are oriented along the electric field due to the input voltage between the transparent electrodes 11 and 22 of the TFT device. Because the light from the back light travels straight in the oriented liquid crystal, the light cannot pass the polarizing film 210. By turning on or off each TFT device on the transparent substrate 1, a color image is displayed on the screen of the liquid crystal display.
FIG. 12 is a plan view of a color filter 21 arranging regularly stripe pixels 211, 212, and 213 of R (red), G (green), and B (blue), respectively. R, G, and B pixels are formed on the transparent electrode 2 by printing or dyeing methods. In the printing method, after preparing a printing ink by mixing a pigment with an ink base and adding desired additives, the stripe pixels are printed on the transparent electrode 2 with the prepared printing ink.
A black matrix is formed at the frame of each pixel of the color filter 21. The black matrix 5 is patterned to form the frame of the pixel 21 by etching a black thin film formed on the transparent substrate 2. As in FIG. 12, the stripe pattern of the black matrix 5 is drawn for the stripe pixel. The width of the black matrix 5 is from 10 .mu.m to a few dozen .mu.m. Since the black matrix 5 is an opaque black line, the black matrix 5 enhances the tone of each pixel 21. As a result, the black matrix provides a distinct tone for each color pixel of the display as a whole.
FIG. 13 is a cross-sectional view of a conventional black film used for the black matrix shown in FIG. 12. The oxides of metals such as chromium (Cr) are used for the black matrix 5 as a black material. The film of the black matrix 5 is a laminated film consisting of a chromium oxide layer 51 and chromium layer 52 thereon on the transparent substrate 2. The laminated film of the black matrix 5 is formed to stack the chromium oxide film layer 51 and chromium film layer 52 in order from the incident direction of external light, for example, sunlight and room light, through the transparent substrate 2.
One of the characteristics generally demanded of the above liquid crystal display is a decrease in reflectance of external light. High reflectance causes the image to blur due to the influence of the reflecting light on the screen of the display. The reflection of the light on the screen occurs on the black matrix which makes up the color filter. Thus, reduced reflectance on the black matrix is desired.
As an index evaluating the reflectance of the black matrix, a bottom reflectance which represents the minimum of the spectral reflectance is employed. The limit of the conventional black matrix is 5% of the bottom reflectance.
For reducing the reflectance of the black matrix, it is known to add carbon to the chromium oxide layer as an additive. According to this known method, the bottom reflectance can be reduced to around 1%. However, some pinholes occur in the black matrix because of localized coagulation of carbon. Thus, this known method significantly detracts from the quality of the liquid crystal display.
In addition, it is known to reduce the thickness of the chromium film layer to reduce the reflectance. In such case, the darkness of the black matrix itself is decreased at the same time because of thinning of the black chromium layer. Such low darkness of the black matrix allows too much light from the back light to transmit the black matrix, so that an enhanced effect of the frame of each pixel of the color filter is offset. As a result, the clearness of the tone on the screen dims in the prior art.